Radio Control Info Articles

Brushless Motor Kv – Measure a Motor’s Kv

What is a Brushless Motor Kv value?

The Brushless Motor Kv value is one of the three important electric motor constants. This motor constant is important as it is one that is used very frequently. When looking for a motor for an RC vehicle, the Kv value is what you will pay attention to. Choosing a motor with the wrong Kv value can ultimately destroy your entire power system, including batteries, ESC and the motor itself. Before motors started using Kv values as a standard, it was typical of motors to show turn values. A turn represent the copper wire making one complete wind within the motors armature. A 2 turn motor would have been a very high strung wind. However it does not tell you anything about the motor, that we would care about in terms of performance.

The definition of the Kv value is how many RPM a motor outputs per every volt. This directly translates in to unloaded RPM for any specific application when the Battery Voltage is known. It is common practice to use Nominal voltage values for the unloaded RPM. This represents the average unloaded RPM throughout a full battery discharge.

The key to this motor constant is to know that the Kv value represents an unloaded condition where the value can vary under load. Unfortunately, the Kv values provided by manufactures vary within a slight range with varying production tolerances and in some cases can be incorrect. For the most accurate real value, it is recommended to measure the Brushless Motor Kv value.

Measuring the Brushless Motor Kv Value

There are a few different ways that the motor Kv can be measured in your own home with common tools. Both methods provide an accurate result.

Data Logger Method – Brushless Motor Kv

This method is rather easy to perform. Hardest part is obtaining the Rm value for the motor from the specification sheet or measuring it yourself.
The tools required in order to use the data logger method are:

Data Logger such as an ESC that has RPM monitoring or an eagle tree system with RPM monitoring.

Electronic Speed Control / ESC

Radio To wire to ESC

LiPo battery to spin motor up to approximately 60% of its rated speed

Formula below / or Calculator tool at bottom of page

This method includes the ESC effects, however with modern ESC these days the results should not be much effected at low current values.

Procedure:

Connect the logging device up according to its instruction manual.

Connect motor and battery to ESC

Power up the system and run the motor up to 100% throttle for a brief moment of time.

Review and Record the following values: RPM, Voltage, current. The 4th value required is the motors internal resistance in one phase. This can be measured as shown in the Measuring Rm page or taken directly from the manufactures provided specifications.

Plug these values into the formula below or use the

Kv = RPM / ( Voltage – ( Current * Rm ) )

Data Logger Method Brushless Motor Kv Calculator

RPM (RPM)

Voltage (V)

Current (A)

Motor Resistance (Rm)

Brushless Motor Kv (RPM/V)

Drill Method – Brushless Motor Kv

This method is a little more difficult to perform depending on the type of drill used.
|The tools required in order to use the drill method are: